collision sensor only detect collision first time only
A collision sensor sends only one positive pulse, as soon as its owner object collides with another object. To make it send pulses as long as this happens, you can use another script to reset the sensor as soon as it fires.

Here is the script I wrote:

import bge

cont = bge.logic.getCurrentController()

sns = cont.sensors['Collision'] # Whatever your sensor is called, not necessarily 'Collision'

sns.reset()


This script should be placed in a Python controller linked to the collision sensor in question (in the Plane object's logic). It will cause the other script to print "Hello" continuously until either 1) the game engine stops or 2) the object no longer collides with anything at all.

I hope this answer helps...